Bead cores based on magnetic material such as ferrite and amorphous magnetic alloys are used as noise suppressors in various electronic circuits for noise suppression purposes. Prior art bead cores include various types, for example, toroidal beads of small-size magnetic material, wired forming type, and axial and radial taping types. These bead cores are directly attached to leads of electronic parts or electrically connected to circuits. In accordance with the size reduction of electronic equipment and the widespread use of equipment to which bead cores are applied, there are acutely increasing needs to reduce the size of bead cores and to provide bead cores in tape form adapted for automatic packaging like conventional parts and in leadless form adapted for surface mounting.
On the other hand, surface mountable multilayer inductors for use as ordinary coils and composite LC parts have been commercially used. Such multilayer inductors are fabricated by alternately stacking magnetic material layers and conductor layers in accordance with thick film techniques, followed by firing.
Coreless and open magnetic circuit type inductors having conductor coil patterns formed on insulating substrates as disclosed in Japanese U.M. Publication No. 25858/1987 and Japanese U.M. Application Kokai No. 78609/1982 are not suitable for such applications because of low impedance whereas multilayer inductors of the closed magnetic circuit type having magnetic material layers can be used as noise suppressing bead cores or noise suppressors.
Although it is desirable to use multilayer inductors as noise suppressing bead cores, elements of reduced size have a lower impedance and the impedance at the service frequency, for example, in the high-frequency range of about 50 to 1000 MHz is insufficient. If the number of laminae or number of turns is increased in order to increase impedance, there results disadvantages including a lower resonance frequency, exacerbated high-frequency response, an increased number of manufacturing steps, an increased cost, and inefficient large-scale manufacture.
The prior art multilayer inductors are generally classified into printed multilayer type and green sheet multilayer type. The printed multilayer type is fabricated, as described in Japanese Patent Publication No. 50331/1985, for example, by printing a conductor pattern of less than 1 turn, printing a magnetic material so that the conductor pattern is partially exposed, and repeating these printing steps, followed by firing.
However, it was found that the printed multilayer type could not use a magnetic material layer of thicker than 0.1 mm because conductor connection becomes uncertain and the impedance at a high frequency of higher than 400 MHz was very low. Even if the number of turns was increased in order to increase impedance, the resonance frequency shifted toward a low frequency side and as a consequence, the high-frequency impedance was low.
On the other hand, the green sheet multilayer type is fabricated, as described in Japanese Patent Application Kokai No. 151211/1989, for example, by forming a conductor pattern on a green magnetic material sheet having a throughhole, and stacking a plurality of such sheets, followed by firing. In this case, a plurality of green sheets are formed with conductor patterns having a predetermined number of turns (less than 1 turn) and stacked such that the conductor patterns are connected through the conductor fillings in the through-holes in the sheets, completing a coil having a predetermined number of turns as a whole. The green sheets at the leading and trailing ends of the coil are provided along opposite edges with extreme lead-out portions of strip shape connected to the coil ends. A pair of external electrodes are connected to the extreme lead-out portions exposed at the opposite edges.
The multilayer inductors for bead cores are required of size reduction to a thickness of about 0.8 to 1.5 mm. In order to achieve a desired impedance with such a size, it is advantageous for large-scale manufacture to reduce the number of layers by forming a spiral coil section on a green sheet such that the number of turns per green sheet is increased to more than 1 turn and increasing the thickness of the green sheet.
In this case, magnetic material sheets used have a thickness of at least 0.2 mm at the end of firing which is greater than in the prior art. Then, a multilayer inductor is fabricated by printing a conductive paste on green sheets in a pattern having a strip-shaped extreme lead-out portion throughout the edge, stacking and compression bonding the printed sheets, firing, and applying an external electrode-forming paste to the opposed edges, followed by firing to form external electrodes. Since the green sheets are too thick to provide wettability with the external electrode-forming paste, insufficient connection can occur between the lead-out portions and the external electrode, leading to the risk of an increase, variation or change with time of DC resistance and even of poor conduction.
In the fabrication of multilayer inductors, it is preferred in view of large-scale production to form an array of many printed patterns of conductive paste each corresponding to the conductor pattern 31 of one layer on a green sheet having a large area, stacking and compression bonding a plurality of such printed sheets, and then cutting the laminate into chips, followed by firing. If the stacking and cutting are done in misalignment, there is increased the possibility of poor conduction as a result of insufficient connection between the external electrodes and the extreme lead-out portions. Misalignment between the patterns resulting from stacking misalignment can also lead to a misalignment between the conductor in the through-hole and the conductor pattern on the underlying green sheet, which also causes losses of manufacturing yield and reliability.